Among the numerous wireless access systems, the core element of the communication facilities is the RF front-end chip. The RF front-end is mainly used for amplifying, frequency converting, filtering and quantizing the weak signal received from the antenna of the receiver, and demodulating it to the baseband signal. Thus, design of the RF front-end circuit shows significant importance to the design of the whole receiver and has direct influence on the performance of the wireless receiving device.
A conventional RF front-end of a communication terminal is structurally constructed of: superheterodyne structure, zero intermediate frequency (IF) structure, double-conversion wide IF structure, double-conversion low IF structure, and the like. Wherein, the superheterodyne structure, due to its optimal sensibility, high selectivity and large dynamic range, is considered to be the most reliable topological structure of receiver and the preferred high-performance receiver. The typical superheterodyne structure, as shown in FIG. 1, employs mixer for converting the HF signal to a lower IF and then going through the channel filtering, amplifying and demodulating, to effectively overcome the problems when dealing with the HF signal. However, an IF filter with high-quality factor is necessity of filtering the image interference effectively, which is unrealizable in the existing CMOS process. Furthermore, as shown in FIG. 2, the IF of the superheterodyne structure is normally lower than the frequency of the RF signal, which leads to the existence of a fatal defect of image interference for the receiver. The superheterodyne structure is commonly applied to the RF front-end of the narrow-band communication system, when applied in the broad-band, such as receiving the 900 MHz RF signal in the 100 MHz to 1.2 GHz range with the superheterodyne structure, providing the IF frequency is of 13.56 MHz. In fact, the receiver receives not only the useful signal at 900 MHz, but also the image interference at 927.12 MHz. The image interference frequency of the RF front-end of the conventional superheterodyne structure totally falls in a narrow range around the useful channel, which has the defects of difficulty in distinguishing the two kinds of signals. The structure has a low sensitivity and is hard to integrate. Besides, when introducing the superheterodyne structure in the broad-band communication, the first local oscillator (LO) is strictly demanded. In the above-mentioned example, the tuning range of the frequency synthesizer is from 113.56 MHz to 1213.56 MHz, which has a lower center frequency and a tuning ratio of 85%.